Differential refractometer

ABSTRACT

This invention measures the change of a fluid&#39;s refractive index with changes in the concentration of a solute dissolved therein. A determination of this quantity is required for many types of chemical analyses especially for the determination of molecular weights. The fluid is restricted to a thin capillary channel (11) within a transparent material (10) such as glass. A fine light beam (18) is incident upon the capillary at an angle close to the critical angle. The axes of the light beam and capillary intersect at a point within the capillary defining thereby a plane within which the refraction occurs. A position sensing (27) device is placed to measure the displacement of the beam twice refracted during its passage through the capillary channel, said measure being used to generate a numerical value of the ratio dn/dc, where dc is the change of solute concentration resulting in a change dn of the solution&#39;s refractive index.

PRIOR RELATED PATENTS AND APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No. 252,331 filed Oct. 3, 1988 and now U.S. Pat. No. 4,952,055.

    ______________________________________                                         Title:          Differential Refractometer                                     Inventor:       Philip J. Wyatt                                                Filing Date:    30 September 1988                                              ______________________________________                                    

The differential refractometer of present invention is directed to a method and apparatus of considerable utility for the determination of the refractive index of fluids that may contain dissolved substances and particularly when such substances result in a change of the refractive index of the fluid. Such instrumentation is of wide use in the chemistry field of high performance liquid chromatography as a concentration monitor.

Expressly incorporated by reference herein are the following related patents and application:______________________________________U.S. Pat. No. 4,490,042Title: Method for Determining the Properties of WineInventor: Philip J. WyattFiling Date: June 4, 1981Issue Date: December 25, 1984U.S. Pat. No. 4,616,927Title: Sample Cell for Light Scattering Measurements.Inventors: Steven D. Phillips, Jeffrey M. Reece and Philip J. Wyatt.Filing Date: November 15, 1984Issue Date: October 14, 1986U.S. Pat. No. 4,710,025Title: Process for Characterizing Suspensions of Small ParticlesInventors: Philip J. Wyatt and Gregory M. QuistFiling Date: September 9, 1985Issue Date: December 1, 1987U.S. Pat. 059,157applicationSer. No.Title: Sample Cell Monitoring SystemInventors: Philip J. Wyatt and Steven D. PhillipsFiling Date: June 5, 1987 Continuation of application 861,863, filed May 12, 1986, now abandoned: this is a division of application Ser. No. 671,181, filed November 15, 1984, now Pat. No. 4,616,927.U.S. Design (Missing)Pat. applicationSer. No.Title: Light Scattering Cell with ManifoldsInventors: Philip J. Wyatt and Richard F. ShuckFiling Date: February 14, 1989______________________________________

DEFINITIONS

The term "light" shall mean electromagnetic radiation.

The term "beam" or "ray" shall mean a pencil of light propagating in a parallel or nearly parallel direction.

The term "beam diameter" of an incident light source with a Gaussian intensity profile, such as a laser, shall refer to the diameter of the beam measured between the points at which the intensity has fallen to 1/e² the intensity at the center of the beam.

The term "forward scattering direction" refers to the range of scattering angles less than 90 degrees with respect to the direction of the incident beam.

The term "backward scattering direction" refers to the range of scattering angles greater than 90 degrees with respect to the direction of the incident beam.

For plane polarized light, the plane perpendicular to the direction of the electric field is called the "V-plane" and said plane polarized light is vertically polarized with respect to said perpendicular plane. The corresponding H-plane is perpendicular to the V-plane and contains the plane wave's incident electric field.

BACKGROUND ART

Measurement of the refractive index of fluids, and especially the changes of said refractive indices when various substances are dissolved in said fluids at different concentrations, is an important requirement for a variety of physical determinations. For example, the determinations of molecular weights of dissolved molecules by light scattering techniques requires the measurement of dn/dc for the dissolved molecules. This differential quantity represents the change of refractive index dn of the solvent with a change of concentration dc of the solute comprised of the molecules to be measured. In the area of liquid chromatography, the concentration of dissolved molecules that have been separated by appropriate columns is determined often by means of a refractive index detector. Such an instrument responds to the changes in fluid refractive index caused by changes in concentration of the solute. These refractive index detection instruments permit thereby the monitoring of solute concentration, an important determination for many forms of liquid chromatography. Another area of application of refractive index determination relates to measurement of sugar content of various fluids. Instruments make these latter measurements based on a determination of the difference of left and right refractive indices of the solution whose sugar content is to be determined. For example, the Swedish company Tecator manufactures such a unit under the Optilab brand name. This technique is described in detail in the U.S. Pat. No. 4,229,105 by Silverbage.

A variety of other techniques for measuring refractive index differences have been developed over the years. Some significant prior art includes the following:

U.S. Pat. No. 4,660,974 by Machler et al. describes the determination of the spectral characteristics of the refractive index of a fluid using white light illumination and examination of the resulting interfering component beams of light. The patent also contains numerous references to other types of interferometric methods.

U.S. Pat. No. 4,569,590 by Karny et al. describes a method of creating a reference Moire pattern with respect to a cell containing a reference fluid. Upon changing the fluid, the pattern changes with said changes being used to determine the refractive index of the changed fluid.

The U.S. Pat. No. 3,619,068 by Broerman describes a refractometer of particular importance for the field of liquid chromatography. It is comprised of two intersecting channels one of which carries the fluid whose refractive index is to be measured. The second channel intersects the first at an angle other than 90°. At the points of intersection the two sections of the second channel are sealed with polished transparent windows. One section contains a light source, the other a light detector. The light detector arm is filled with optical rods whose faces closest to the fluid are oriented so that the light transmitted through the fluid strikes the rods at approximately the critical angle of the fluid. As the refractive index of the fluid changes, the amount of light reaching the detector will change, permitting thereby the deduction of said fluid refractive index change.

DISCLOSURE SUMMARY OF THE INVENTION

It is the major objective of my invention to monitor the refractive index of a fluid stationary within or flowing through a capillary channel surrounded by a transparent medium. This is accomplished by detecting the deviation of a fine beam of light after it has passed through the capillary, said displacement from the incident beam being due to the difference between the refractive index of the fluid and the transparent medium surrounding it. The transparent medium surrounding the capillary is of a refractive index higher than that of the fluid and the passage of the light beam is from said transparent medium into the capillary at angle obtuse to the capillary axis and then exiting the capillary back into said transparent medium. The deviation of the light beam from its straight line path is due to the differences of the corresponding refractive indices of the fluid and surrounding medium. The deviation will be greatest as the incident beam impinges on the capillary at an angle (measured with respect to the normal to the capillary axis) slightly less than the critical angle.

It is a further objective of my invention to provide a means for measuring the change of refractive index of a fluid at essentially the same physical position from which the fluid's light scattering properties are measured. This spatially coincident measurement is particularly important in the field of liquid chromatography, especially when applied to the technique of size exclusion or gel permeation chromatography.

Another objective of my invention is the deduction of the concentration of dissolved substances in the fluid, since a changing concentration of a dissolved solute will cause corresponding changes in the refractive index of the resulting solution. Such changes in concentration can be quantitated by converting said beam deviation into corresponding changes in concentration.

My invention will be particularly useful when applied to a light scattering cell similar to those described in the above referenced U.S. Pat. No. 4,616,927 of which I am a co-inventor. Said patent, hereinafter called the "927" patent, describes an important type of light scattering cell permitting measurement of the light scattering properties of solutions with minimal interference from light scattered at the cell interfaces.

tted through the fluid strikes the rods at approximately the critical angle of the fluid. As the refractive index of the fluid changes, the amount of light reaching the detector will change, permitting thereby the deduction of said fluid refractive index change.

DISCLOSURE SUMMARY OF THE INVENTION

It is the major objective of my invention to monitor the refractive index of a fluid stationary within or flowing through a capillary channel surrounded by a transparent medium. This is accomplished by detecting the deviation of a fine beam of light after it has passed through the capillary, said displacement from the incident beam being due to the difference between the refractive index of the fluid and the transparent medium surrounding it. The transparent medium surrounding the capillary is of a refractive index higher than that of the fluid and the passage of the light beam is from said transparent medium into the capillary at angle obtuse to the capillary axis and then exiting the capillary back into said transparent medium. The deviation of the light beam from its straight line path is due to the differences of the corresponding refractive indices of the fluid and surrounding medium. The deviation will be greatest as the incident beam impinges on the capillary at an angle (measured with respect to the normal to the capillary axis) slightly less than the critical angle.

It is a further objective of my invention to provide a means for measuring the change of refractive index of a fluid at essentially the same physical position from which the fluid's light scattering properties are measured. This spatially coincident measurement is particularly important in the field of liquid chromatography, especially when applied to the technique of size exclusion or gel permeation chromatography.

Another objective of my invention is the deduction of the concentration of dissolved substances in the fluid, since a changing concentration of a dissolved solute will cause corresponding changes in the refractive index of the resulting solution. Such changes in concentration can be quantitated by converting said beam deviation into corresponding changes in concentration.

My invention will be particularly useful when applied to a light scattering cell similar to those described in the above referenced U.S. Pat. No. 4,616,927 of which I am a co-inventor. Said patent, hereinafter called the "927" patent, describes an important type of light scattering cell permitting measurement of the light scattering properties of solutions with minimal interference from light scattered at the cell interfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates Snell's Law for the refraction and reflection of a ray of light.

FIG. 2 shows the structure of a standard split prism cell used to measure the refractive index difference of two fluids.

FIG. 3 shows the refractions of a ray traversing a liquid capillary travel within a transparent medium.

FIG. 4 shows a detail of the refractions responsible for the beam deviations arising from positive and negative refractive index increments of the fluid within a capillary.

FIG. 5 shows a top view of the capillary-refracted rays' passage through the curved surface of a refraction cell described in "927" patent.

FIG. 6 shows the liquid capillary acting as a diverging cylindrical cell with respect to the incident light beam of finite cross section.

FIG. 7 shows a refraction cell of "927" patent.

FIG. 8 shows a spherical refraction cell permitting light beam incidence normal to curved surfaces outside of a plane containing a set of detectors for the measurement of scattered light for the same illuminated volume element.

FIG. 9 shows the results of an early experimental measurement confirming the sensitivity of the present invention.

MODES FOR CARRYING OUT THE INVENTION

FIG. 1 presents a diagram of the refraction and reflection of a light ray 1 in a transparent medium of refractive index n₁, striking the plane interface 4--4 at an angle θ₁, and entering a medium of refractive index n₂. The ray 2 is reflected at an angle θ₁ and the ray 3 is refracted into medium 2 at an angle θ₂. The relationship between the angles θ₁ and θ₂ is given by Snell's Law

    n.sub.1 sin θ.sub.1 =n.sub.2 sin θ.sub.2.      (1)

Snell's Law was derived by Huygens in the 17th Century from his wave description of light and more rigorously may be derived directly from Maxwell's equations. The resultant equations of refraction and reflection between media for which the refractive indices may be complex are often referred to as the Fresnel equations. FIG. 1 has been drawn for the case where n₂ >n₁. Consider for that case, the situation where the ray 1 just grazes the plane interface, i.e.

    θ.sub.1 =π/2.                                     (2)

Equation (1) may be rewritten, therefore, as

    θ.sub.2 =θ.sub.c =sin.sup.-1 (n.sub.1 /n.sub.2).(3)

Equation (3) so-written defines the critical angle θ_(c) so-named because reversing the ray direction from 3 defines an angle of incidence at the plane 4--4 from medium n₂ which represents the limiting angle in that medium. Any ray incident on the interface 4--4 from medium n₂ at an angle greater than θ_(c) will be perfectly reflected and no energy will be refracted into medium n₁. At the critical angle, a surface wave is said to be launched between the media and that surface wave is exponentially attenuated in the medium n₁. These surface or evanescent waves have been shown to have most interesting properties and are of importance for a number of devices and inventions in common use. A textbook by N. J. Harrick on "Internal Reflection Spectroscopy" on his article in volume 17 of Applied Spectroscopy (1987) should be consulted for some further explanations and interesting applications.

FIG. 2 shows the structure of a conventional split prism device used to measure the difference of the refractive indices of two liquids contained, respectively, in triangular shaped regions 5 and 6. The incident ray 1 passes through medium 5, strikes the interface at 8 and is refracted in medium 6. The refracted ray leaves medium 6 at 9 where it appears displaced and parallel to the undeviated ray 7. For small refractive index differences between the liquids, it may be shown that

    d=L(n.sub.1 -n.sub.2)/(2n.sub.1)                           (4)

where L is the width of the square structure. Note that the change in the displacement d is directly proportional to the difference of the refractive indices of the solutions.

With these preliminaries, the details of my invention will be clearly understood by those skilled in the art. FIG. 3 presents the refraction of a ray of light passing through a liquid containing capillary channel. The ray 1 begins in transparent medium 10, strikes a capillary channel 11 at the point 12, is refracted by the difference of refractive indices between the medium 10 and liquid 11, strikes the opposite side of the capillary, and is refracted again as the ray 3 emerges at 13 into the medium 10 once again. The path of the indeviated ray 14 is indicated by the dotted line. The deviation, d, of the refracted ray with respect to the undeviated ray is also shown. For this figure, the geometry corresponds to all rays and the capillary lying in the same plane. If the refractive index of 11 were the same as that of the surrounding transparent medium 10, the beam 3 would be undeviated and superimposed on the undeviated ray 14.

Let us now examine further the case where the liquid is of a refractive index less than the surrounding medium. For ease of discussion, we shall consider the transparent medium to be glass and the liquid to be a transparent fluid such as water or toluene or tetrahydrofuran or other so-called mobile phases used in forms of liquid chromatography. Alternatively, these liquids are more commonly called solvents. This glass/mobile phase dichotomy is intended for illustrative purposes only. There are many other transparent solids and liquids by which means this invention is equally operative. For the discussion and explanation that follows we shall assume

    n.sub.g >n.sub.s                                           (5)

where n_(g) is the glass refractive index and n_(s) is the refractive index of the solvent. The direction of the displacement d of FIG. 3 is characteristic of the relation of Eq. (5). As the refractive index of the liquid (solvent) increases because of the increased concentration of dissolved substances (solutes), the displacement of the refracted ray 3 will decrease and the refracted ray will approach co-linearity with the undeviated ray as n_(s) approaches n_(g). Since, characteristically, the capillary channel will be of finite length, it is important to select the incident angle so that the ray displacement is not so great as to remove the ray completely, or so that it strikes ends of the capillary or the supporting structure. For example, if the angle of incidence from the glass was the critical angle per Eq. (3), then there would be no capillary traversing ray whatsoever.

Referring now to FIG. 4, we see the refractions and displacement of the ray 1 as it passes through the capillary 11, beginning and ending in the glass 10. The capillary channel is shown of diameter 2r in the plane containing all rays. The incident ray 1 makes an angle θ' to the capillary axis. For this geometry, the conventional angle of incidence is π/2-θ'. The undeviated ray would appear as ray 14, ray 3 corresponds to the ray displaced by the pure solvent, and ray 15 displaced a distance Δ from ray 3 toward 14 is the ray corresponding to changes occurring due to a dissolved solute in the solvent causing a refractive index change Δn.

The various distances to the right of the perpendicular through the point of incidence 12 are labeled in the figure. Combining these results, the displacement Δ transverse to the ray 3 may be expressed simply in the form

    Δ=2r(cot θ.sub.2 -cot θ.sub.1) sin θ'.(6)

The angles θ', and θ₁ are related by Snell's Law

    n.sub.g sin (π/2-θ')=n.sub.s sin (π/2-θ.sub.2)(7)

    or

    n.sub.g cos θ'=n.sub.s cos θ.sub.2.            (8)

Further

    cot θ.sub.2 =cos θ.sub.2 /(1-cos.sup.2 θ.sub.2).sup.1/2=( n.sub.g /n.sub.s) cos θ'/[1-(n.sub.g /n.sub.s).sup.2 cos.sup.2 θ'].sup.1/2                                         (9)

and similarily, ##EQU1## Equation (6) may be readily evaluated given θ',n_(g), Δn and n_(s). Note furthermore that

    θ'>cos.sup.-1 (n.sub.s /n.sub.g),                    (11)

since the right hand side of Eq. (11) corresponds to π/2-θ_(c), where θ_(c) is the critical angle of Eq. (3). If θ' is less than or equal to cos⁻¹ (n_(s) /n_(g)), then no ray enters the capillary 11.

In the limit ##EQU2## Eq. (6) may be written in the approximate form ##EQU3##

Note that Eq. (13) confirms a linear displacement change, Δ, with a linear refractive index change Δn, since θ', r, p, and n_(s) are fixed quantities. For Δn much above 10⁻⁴ and θ' within 1° of the critical angle, this linear variation requires correction to second order.

FIG. 5 shows a cross sectional view of the cylinder cell 10 of the "927" Patent. The capillary 11 is bored along a cylinder diameter and polished. In its conventional configuration, a light source is incident through the capillary fluid from 16, exiting at 17. Scattered light detectors are placed circumfrentially about the cylinder and lie in the plane of the capillary. These detectors are aimed toward the center of the capillary, and placed with faces parallel to the cylinder curved surface closest to them. Details are presented in the "927" Patent. For purposes of the present refractometer invention, a secondary source of light 18 lying in the plane of the detectors, or otherwise above or below such plane, is directed toward the center of the capillary through which it would pass were it not for the refractions of the liquid within the capillary 11. The path of an undeviated ray is shown at 14, while the fluid has created the ray displaced therefrom by the liquid at 13. As the refractive index of the fluid within the glass capillary increases, the refracted ray will emerge at 15 and move towards coincidence with 14.

We will now examine Eqs. (6) and (10) for angles satisfying Eq. (11). As an example, let us take n_(g) =1.61655 and n_(s) =1.3333. Here n_(g) is the refractive index of Hoya F2 glass at a wavelength of 632.8 nm. Such glass is of frequent use in optical fibers as well as the refraction cells of my co-invention "927" Patent. F2 glass is commonly manufactured by Schott and Hoya Glass companies, among others. Let the solvent refractive index, n_(s), correspond to water, a common solvent/mobile phase in liquid chromatography, and let the incident wavelength be 632.8 nm. This is the wavelength of the most common He-Ne laser. For the stated glass and liquid, the incident ray must strike the capillary at an angle θ' greater than

    cos.sup.-1 (1.333/1.61655)=34.4°.                   (15)

For purposes of subsequent calculation, we shall take the diameter of the capillary, 2d, to be 1.5 mm, a commonly found dimension of the refraction cell above referenced.

Table 1 presents the displacement in mm of the refracted ray 3 of FIG. 4 from the undeviated ray 14 for the aforementioned values of n_(g) and n_(s). Also listed are the calculated transmittances of the ray 3 for vertical and horizontal polarizations of the incident ray. Vertically polarized light corresponds to plane polarized light for which the electric field is polarized perpendicular to the plane of the figure. Horizontally polarized light corresponds to plane polarized light whose electric field lies in the plane of the figure. The light transmitted through the capillary channel into the glass along ray 3 has a transmittance T that may be derived from the Fresnel relations presented, for example, by C. W. Ditchburn in his book "Light." At each glass/liquid interface, there is a reflected fraction R of the incident power. Thus, the light transmitted through the capillary is

    T=(1-R).                                                   (16)

The reflectance R for the case of horizontally polarized light (electric field lies in the plane of the capillary channel axis) is

    R.sub.H =tan.sup.2 (θ'-θ.sub.1)/tan.sup.2 (θ'+θ.sub.1).                                 (17)

For vertically polarized incident light (electric field polarized perpendicular to the capillary channel axis),

    R.sub.v =sin.sup.2 (θ'-θ.sub.1)/sin.sup.2 (θ'+θ.sub.1).                                 (18)

                  TABLE 1                                                          ______________________________________                                         Ray displacement in mm for water filled                                        capillary channel in F2 glass.                                                 Θ' (degrees)                                                                        Displacement  T.sub.V T.sub.H  (%)                                  ______________________________________                                         35         6.10          24      40                                            36         3.23          45      67                                            37         2.30          58      79                                            38         1.81          66      86                                            39         1.49          72      91                                            40         1.27          76      94                                            45         0.71          88      99                                            ______________________________________                                    

Note that the closer the incident ray 1 lies to the critical angle of 34.4°, the greater is the displacement and the smaller is the transmitted fraction for either polarization.

Next, consider the effect of a refractive index change in the capillary fluid of 0.0005, i.e. let the liquid refractive index increase to 1.3338. This is a change that might be encountered for a peak concentration of 5 mg/ml in a size exclusion chromatography experiment. The utility of a differential refractometer is directly related to its ability to monitor such changes in refractive index. The calculations of Table 2, which follow, include another column, viz. the displacement change Δ with respect to the position of the pure water displacement.

                  TABLE 2                                                          ______________________________________                                         Ray displacement and displacement changes (in mm), Δ,                    caused by a capillary solvent (water) refractive index                         increase of 0.0005 in F2 channel glass.                                        Θ' (degrees)                                                                       Displacement                                                                               Δ T.sub.V  (%)                                                                          T.sub.H  (%)                              ______________________________________                                         35        5.90        0.194   25     42                                        36        3.19        0.043   46     67                                        37        2.28        0.021   58     80                                        38        1.79        0.013   66     87                                        39        1.48        0.009   72     91                                        40        1.26        0.007   76     94                                        45        0.70        0.002   88     99                                        ______________________________________                                    

Again we see that the most pronounced effects occur closest to the critical angle.

Consider now a second example where instead of F2 glass, we choose Schott K5 glass whose refractive index at 632.8 nm is 1.52064. The critical angle (measured with respect to the capillary axis) is now 28.74° per Eq. (11). Table 3 presents the displacement in mm of the refracted ray 3 of FIG. 4 from the undeviated ray 14 for the case of water in a K5 cell.

                  TABLE 3                                                          ______________________________________                                         Ray displacement in mm for water filled                                        capillary channel in K5 glass.                                                 Θ' (degrees)                                                                        Displacement  T.sub.V T.sub.H  (%)                                  ______________________________________                                         29         8.98          16      24                                            30         3.44          46      60                                            31         2.30          60      75                                            32         1.75          69      84                                            33         1.42          76      89                                            35         1.03          83      94                                            40         0.58          92      99                                            ______________________________________                                    

Introducing a refractive index change of 0.0005 now yields the values shown in Table 4:

                  TABLE 4                                                          ______________________________________                                         Ray displacement and displacement changes (in mm), Δ,                    caused by a capillary channel solvent (water) refractive index                 increase of 0.0005 in K5 glass.                                                Θ' (degrees)                                                                       Displacement                                                                               Δ T.sub.V  (%)                                                                          T.sub.H  (%)                              ______________________________________                                         29        8.28        0.698   18     26                                        30        3.37        0.071   46     61                                        31        2.27        0.030   61     76                                        32        1.74        0.017   70     84                                        33        1.41        0.012   76     89                                        35        1.02        0.007   84     94                                        40        0.58        0.003   92     99                                        ______________________________________                                    

As a final example, consider the solvent toluene whose refractive index is about 1.49, in F2 glass. The critical angle from Eq. (3) is now 22.82 degrees. Tables 5 and 6 present the corresponding data.

                  TABLE 5                                                          ______________________________________                                         Ray displacement in mm for toluene filled                                      capillary channel in F2 glass.                                                 Θ' (degrees)                                                                        Displacement  T.sub.V T.sub.H  (%)                                  ______________________________________                                         23         10.04         15      19                                            24         3.18          51      60                                            25         2.06          66      75                                            26         1.54          75      84                                            27         1.24          81      89                                            30         0.76          90      96                                            35         0.44          95      99                                            ______________________________________                                    

                  TABLE 6                                                          ______________________________________                                         Ray displacement and displacement changes (in mm), Δ,                    caused by a capillary channel solvent (toluene) refractive                     index increase of 0.0005 in F2 glass.                                          Θ' (degrees)                                                                       Displacement                                                                               Δ T.sub.V  (%)                                                                          T.sub.H  (%)                              ______________________________________                                         23        8.81        1.228   18     22                                        24        3.10        0.084   52     61                                        25        2.03        0.034   67     76                                        26        1.53        0.020   76     84                                        27        1.22        0.013   81     89                                        30        0.75        0.006   90     96                                        35        0.44        0.003   96     99                                        ______________________________________                                    

Measurement of the displacement change, Δ, may be achieved quite easily. Using a split photodiode, for example, of the type manufactured by United Detector Technology, changes less than 0.1 micrometer may be detected. Such a refractive change would correspond to a concentration change of the order of 10⁻⁴ ng/ml. Referring to the examples presented, we note the following:

1) The greatest displacement changes occur for incident light beam angles approaching the critical angle. The displacement change becomes smaller quite rapidly as the incident beam moves away from this critical angle.

2) The displacement change increases with increasing glass/liquid refractive index difference.

3) The transmission change, for both V and H incident polarizations, dT/dθ', decreases as the liquid refractive index increases.

Note that a narrow beam of light of finite cross section X passing through a capillary channel of circular cross section will produce a flaired oblong refracted beam transverse to the refraction plane upon emerging from the capillary structure of FIG. 5. FIG. 6 presents a side view of a finite beam of diameter X passing through a capillary whose refractive index is less than that of the surrounding medium. Since the capillary refractive index is less than that of the glass, the capillary will act as a negative cylindrical lens. A split photodiode detector would be placed preferentially with its face nearly perpendicular to the emerging beam and its division axis along the flair direction.

FIG. 7 is a drawing of the cylinder refraction cell and manifold structure described in the "927" Patent. The cylindrical glass cell 10 is shown with the fine capillary channel 11 attached at either end by inlet and outlet manifolds 30 and 17, respectively. The fluid sample is introduced at 31, pumped into the cell, and leaves the cell at 19. Each manifold is fitted with a laser beam entrance or exit window, 20 or 21, respectively. The laser source is incident from 22 used for scattering experiments produces a beam entering at 20, passing along the capillary 11 axis, and exiting through window 21. An array of detectors, not shown, lie circumferentially about the cylinder cell in a plane parallel to the cylinder ends and coplanar with the capillary channel 11. Other manifold configurations include an integrated structure such as illustrated by the design patent application by Wyatt and Shuck referenced earlier.

The actual shape of the transparent region through which the capillary passes may be of many different forms, each form selected on the basis of the geometry and function of the region into which the differential refractometer is to be located. For example, for one preferred embodiment of the invention, the glass region would be in the form of the refraction cell disclosed in the previously referenced "927" Patent by Phillips et al. and shown schematically and in cross section in FIG. 5. However, such an orientation of the incident light beam to the capillary axis could be effected equally well by positioning the source 18 above the scattering detection plane 23 by using a spherical glass cell 24 in the manner indicated in FIG. 8, the preferred embodiment of the invention. Here the light beam 18 is incident on the surface at 28, passes through the capillary 11, is refracted thereby as has been discussed earlier, and then emerges through the opposite surface at 26 to be detected at 27. Note that the position sensing detector 27 and the incident light source 18 lie out of the conventional detection plane 23 of the aforereferenced "927" patent.

The refractometer light source 18 should be contrasted with the second light source 22 whose beam passes through and parallel to the capillary 11 and scatters from the solution entrained therein. While the light source 22 and capillary 11 lie in the scattered light detector plane 23, the refractometer light source 18 and the corresponding position sensing detector 27 lie in general in a different plane containing also the capillary axis 11.

Although the preferred and most common configuration of the capillary will be of cylindrical cross section, other forms may be used to equal advantage. For example, the cross section could be rectangular, being formed by combining planar polished transparent elements to yield a capillary void through which the fluid may flow, or in which it may be placed, of rectangular cross section. Rectangular boxes may be drilled easily by means of ultrasonic machining such as performed by machines manufactured by Bullen Ultrasonics of Ohio. In addition, the capillary need not be straight since a slightly curving capillary, whose radius of curvature is large compared to its transverse dimensions, may be useful for certain types of geometries.

Having described the elements of my invention together with some examples of glass-capillary structures that might be employed therein, I now discuss further some preferred embodiments of the invention.

The ability to determine the refractive index change of a fluid in which a solute has been dissolved requires the following elements:

A) A collimated light beam oriented with respect to the fluid bearing capillary channel 11 at an angle somewhat less than the critical angle defined by the refractive indices of the entraining transparent medium (higher refractive index) and the solute-absent fluid (lower refractive index) given by Eq.(11). Preferentially, the illuminating light beam will be a laser unpolarized or polarized. Incident polarization can be plane or circular or combination thereof;

B) Attachment means whereby the light beam of A may be positioned with respect to and rigidly attached to

C) A transparent refraction cell containing a finely polished capillary channel carrying said fluid therethrough; and

D) Position sensing means whereby the displacement of the refracted transmitted beam caused by a fluid refractive index change may be detected and quantified.

It is important to note that if the solute has a negative refractive index increment dn/dc with concentration, the angle of the incident beam preferentially would be oriented at a smaller angle than if dn/dc is positive. This will prevent the refracted beam from approaching the critical angle too closely thereby preventing any light from being refracted. For the Broerman device, described earlier, this type of an adjustment cannot be effected. In addition, the Broerman device is directed to detecting intensity differences whereas the present device detects both displacement and intensity variations with most emphasis on the former.

In a preferred embodiment of the invention, the refraction cell would be of a spherical shape and form claimed in the "927" patent. The light source would be a fine laser beam oriented normal to the spherical face, and entering the fluid bearing capillary channel at an angle (measured with respect to the normal to the capillary channel axis) slightly less than the critical angle defined by Eq.(3). After refraction through the capillary channel in the manner shown in FIG. 4 and emerging through the opposite face of the sphere as detailed in FIG. 8, the beam is centered upon a position sensing element 27. The reference position of the beam with respect to said sensing element preferentially corresponds to the refracted position 3 of FIG. 4 caused by the pure solvent alone. Any change in refractive index of the solvent would cause a shift of the beam 15 from said reference position 3 which in turn would cause a change in the output signal of the position sensing device, such as shown at 27 in FIG. 8.

The output signal could be generated in different manners. For example, with a split photodiode of the type described previously, each half of the photodiode would be subjected to different levels of incident light flux caused by changes in the centering of the beam as well as intensity variations transverse to the beam due to different transmissivities of the refracted elements of the beam arising from its finite cross section and the corresponding slight differences of incident angles. A typical output would be generated by the difference of the two corresponding output currents or voltages. Since light sources of considerable short term stability are required for increased precision, the difference signal could be referenced to the light power incident on the structure. Said incident light power may be obtained from a reference beam incident on a different photodiode. For example, as many lasers have a small amount of light energy leaking out through their rear laser mirror, said light being easily detected by the simplest of photodetectors fastened to the rear of the laser. As the laser output varies, so too would the rear beam power. Alternatively incident reference power could be monitored with respect to a beam splitter at the emitting forward end of the laser. Such monitoring systems have been described in considerable detail in the copending application earlier referenced in this description. Another type of dual output device is a continuous photodiode structure manufactured by Silicon Detector Corporation. This device produces an output at each of its ends proportional to the distance from the center of the device of the beam.

Note that the differential response of the position sensor as well as the light beam monitor should be converted to a numerical or analog representation for practical use. Thus a preferred output of the instrument would be a numerical representation of the change of refractive index of the solution relative to a previously measured or stored reference value. The most accurate values so-displayed would be generated internally or by external computer means to yield a value dn proportional to the ratio of the differential detector output divided by the incident light beam intensity. Since Eq. (6) suggest a proportionality which generally may not be quite linear as shown in Eq. (13), the proportionality factor as a function of Δ may be stored in computer means connected to the instrument for subsequent reference, may be generated and stored by means of a calibration procedure, and/or may be generated by external computer means. If the unit is to be used on-line, for example with a simultaneous light scattering measurement as shown in plane 23 of FIG. 8, then the analog signals generated by the instrument would be transmitted directly into an analog-to-digital multiplexing converter for on-line use in another preferred embodiment of the invention.

Since the displacement, Δ, is a monotonic function of the refractive index change, dn, and said refractive index change will be directly proportional to the concentration change of the dissolved substance, the said displacement may be referred directly to the actual concentration of said dissolved substance. Thus referred to the displacement, d, of the pure liquid, the measured displacement which induces a monotonic response in a detecting position sensing means may be used to generate a solute concentration value, c. Each displacement will correspond, therefore, to a specific concentration from the relation

    n=n.sub.o +(dn/dc)c,                                       (19)

    or

    n-n.sub.o =Δn=(dn/dc)c                               (20)

    and

    c=Δn/(dn/dc),                                        (21)

where n_(o) is the refractive index of the pure fluid absent dissolved substance and n is the refractive index of the fluid plus solute. dn/dc is easily measured by making a measurement of Δn caused by a concentration change Δc, then extrapolating to the limiting value of Δn/Δc, i.e.

    (dn/dc)=lim (Δn/Δc)as Δc→0        (22)

FIG. 9 shows the result of an early experimental confirmation of the invention. A 5 W plane polarized laser beam was attached to a structure holding a cylindrical refraction cell of the type described in the "927" patent. The wavelength of the laser was 632.8 nm, the cell refractive index was 1.61655, and the capillary was filled with water of refractive index 1.333. Injected into the capillary and pumped therethrough by means of a 510 chromatography pump of the type manufactured by the Waters Company was a 20 microliter aliquot of a 0.1% dextran (in NaCl buffer) of approximate molecular weight 600,000. The change of refractive index with respect to concentration, dn/dc, was about 0.10. The difference of the two signals produced by the United Detector Technology split photodiode was amplified to produce the plot of FIG. 9 of voltage versus time. Since the pump was operating at a speed of 0.5 ml/minute, the time scale also corresponds to the so-called retention volume obtained by multiplying time by the factor 0.5 ml/min. The small spikes superimposed on the curve arose from pump pulsations causing the structure to move slightly relative to the laser and detector source. These are easily removed by incorporating said refraction cell elements into a stationary structure. The breadboard configuration used to make these early measurements was assembled without particular attention to structural stability.

Although the impressive results of FIG. 9 were generated using a continuous wave laser, further signal enhancement would be achieved by modulating the laser beam by electrical or chopper means. At a frequency above the cutoff frequency of the standard multi-angle detectors of the "927" device, the modulated refractive index signal would not be detected by the multi-angle detectors. Thus, the multi-angle scattering as well as the refractive index changes could be monitored simultaneously even though both were performed at the same wavelength using different laser sources. Alternatively, both sets of measurements could be interspersed, turning one laser off while the other was on. A further variation of the dual measurement could be perform using light sources of different wavelengths in combination with filters covering the multi-angle scattering detectors and permitting thereby the selective detection of the appropriate signals with a narrow band pass filter covering the position sensitive detector corresponding to the wavelength of the refractometer light source and covering the multi-angle scattered light detectors with narrow band pass filters at the wavelength of the scattering light source. Many variations of these detection strategies will be clearly evident to those skilled in the art of light detection from sources fixed in space.

For each type of fluid of different refractive index, there will be a unique optimal angle at which the incident light beam should be set. Thus, unless said fluid be always the same, means should be provided to adjust the angle of the incident beam with respect to the capillary. Optimally, this angle will lie close to the critical angle, perhaps one or two degrees therefrom. Alternatively, the light source may be fixed and the capillary channel containing refraction cell rotated so that the beam/capillary channel angle setting may be adjusted thereby. Means should also be provided, as required, to orient the position sensing element that monitors the position of the refracted beam. Once the light beam/capillary channel angle has been set for the solvent selected, the adjustment of the position sensing element is straightforward: with the capillary channel filled with the pure solvent, the position sensing element is set at a position that will yield a null electrical signal. Alternatively, it could be placed at a position yielding a small signal relative to those it would generate with increasing solute concentration. Depending upon the actual physical structure of the position sensing element, there could be many similar orientations thereof, as will be readily apparent to those skilled in the art.

In closing this discussion of my invention, some further features are disclosed. The light beam, which would be preferably a laser beam, should be of a cross section less than the cross section of the capillary channel since only those elements of the beam striking the capillary will be refracted and, in a preferred embodiment, all elements of the incident light beam should be affected by refraction in the capillary channel. Further, as has been alluded to earlier, the best orientation of the capillary channel and light beam should be such that their center lines intersect. Any other orientation would correspond to a partial grazing by the incident beam. The symmetric spatial intersection of the two axes will insure further the symmetrical refraction of the incident light beam by the capillary channel with respect to the plane defined by these two lines.

We note, furthermore, from Eq. (6) that the displacement of the refracted beam, Δ, with respect to the incident beam, increases as the capillary channel diameter, 2r, increases. The capillary channel itself, therefore, must be long enough to accommodate both the refracted beam as well as the internally reflected beam without said beams striking the capillary terminations or any other similar obstacles. Such obstacles would result in secondary reflections by said beams and these reflections are generally unwanted sources of stray light. Since the sensitivity of the refractometer increases as the diameter of the capillary, the optimal cell may be larger than the standard refraction cell, which is about 22 mm. However, from the chromatography point of view, a larger capillary volume will result in a greater band broadening of the solute contained therein. Thus a judicious tradeoff of design parameters must be made.

In this preferred embodiment of the invention, the refractometer structure would serve a dual purpose as indicated in FIG. 8. Surrounded by detectors 27 coplanar with the capillary 11 and illuminated through the capillary by a light source 22, the structure will function by measuring the angular variation of light scattered by the particles flowing through the capillary according to the cited "927" patent. Adding the second light source 18 and displacement detector 27, the structure will function as a concentration sensitive detector as described in this present application. In order that neither light source interfere with the detectors associated with the other, it may be necessary to turn one off while the other is being used. This may be achieved by mechanical, shutter means or, in the case of certain classes of lasers, the light sources themselves may be modulated. In either event, the time delay between such successive measurements may be reduced to a time period small relative to the displacement of the particles entrained in the fluid flowing through the capillary 11. In this manner, the measurement of the scattered light variations with detector angular position from an ensemble of particles/molecules may be measured from essentially the same physical particles whose concentration is simultaneously monitored. This may be achieved with the preferred embodiment of the invention, as shown in FIG. 8.

INDUSTRIAL APPLICABILITY

This invention of a flow through differential refractometer will have significant industrial application for chemistry and in particular polymer chemistry wherein the physical characteristics of large molecules are sought. Combined with the technique of size exclusion chromatography the preferred embodiment of my invention will permit the measurements of molecular size and weight of each separated fraction irrespective of the constancy of flow rate since both light scattering and concentration measurement may be performed on the same flowing volume element of effluent. Used with another form of concentration detector, such as ultraviolet absorption or evaporative mass detection, the invention will permit deduction of the differential refractive index increment dn/dc with concentration. In this manner, the physical parameters of co-polymers may be derived by combining the measurements of differential refractometer, light scattering array, and concentration sensitive detector.

While there has hereinbefore been presented what is at present considered to be the preferred embodiments of the method and apparatus for measuring the refractive index changes within the fluid bearing capillary, it will be apparent to those of ordinary skill in the art that many modifications and variations may be made therefrom without departing from the true spirit and scope of the invention. All such variations and modifications, therefore, are considered to be a part of the invention. 

What is claimed is:
 1. A differential refractometer for measuring the refractive index change of a fluid, caused by a change in the concentration of a solute dissolved therein, comprised ofA. a fluid bearing capillary lying along a diameter of a spherical transparent medium, said medium being of a refractive index greater than that of said capillary borne fluid; B. a light source producing a fine light beam coplanar with said capillary, said beam being directed to pass through said transparent medium and to intersect said fluid bearing capillary causing thereby said light beam to pass from said transparent medium into said capillary borne fluid and then back out into said transparent medium after thereby having been twice refracted by said fluid in said fluid bearing capillary; C. a refracted beam detector element receiving said twice refracted beam and which is responsive to the displacement of said twice refracted beam caused by refractive index change of said fluid due to said dissolved solute and producing a signal corresponding to said displacement; D. conversion means whereby said refracted beam displacement signal may be converted into a direct numerical or analog representation of dn/dc where dn is the refractive index change of said fluid caused by a concentration change dc of said solute within said fluid.
 2. The refractometer of claim 1 where said light beam is oriented with respect to said capillary at an angle slightly less than the critical angle θ_(c) where

    θ.sub.c =sin.sup.-1 (n.sub.s /n.sub.g),

n_(s) is the refractive index of said capillary borne fluid, and n_(g) is the refractive index of said transparent medium.
 3. The refractometer of claim 1 where said fine light beam is of a diameter less than the diameter of said capillary.
 4. The refractometer of claim 1 where said fine light beam is generated by a laser.
 5. The refractometer of claim 1 where said fine light beam may be adjustably fixed at different angles of incidence to said capillary.
 6. The refractomer of claim 1 where said refracted beam detector element is a two element photodiode.
 7. The refractometer of claim 1 where said refracted beam detector element is adjustably fixed so as to intercept said twice refracted beam passing through said capillary.
 8. The refractometer of claim 1 where said fluid bearing capillary is provided with liquid handling means permitting the injection of fluid therethrough.
 9. The refractometer of claim 1 where said refractometer incorporates a second light source producing a second fine light beam passing parallel to and through the center of said capillary within said sphere, said transparent medium being further surrounded by an array of detectors such that said detectors, capillary, and second fine light beam lie essentially in a plane; said detectors detecting light scattered from said capillary borne fluid over a range of scattering angles.
 10. The refractometer of claim 9 where said array of detectors lies in a plane essentially orthogonal to said plane defined by first light beam, capillary, and twice refracted beam detector.
 11. The refractometer of claim 1 where said fine light beam is monitored by a signal proportional to the incident intensity of said light beam and said displacement value is divided by said signal prior to conversion into said fluid refractive index change.
 12. The refractometer of claim 7 where the position of said refracted beam detector element is adjustably set such that the signal produced thereby is null when said dissolved solute is absent from said fluid.
 13. The refractometer of claim 1 where said refracted beam detector element incorporates an absolute intensity monitor relative to the intensity incident on said transparent medium from said fine light beam.
 14. A method for measuring the refractive index change of a capillary borne fluid caused by a change in the concentration of a solute dissolved therein, comprising the steps ofA. providing a fluid bearing capillary lying along a diameter of a spherical transparent medium, said medium being of refractive index greater than that of said capillary borne fluid; B. illuminating said capillary by a coplanar fine light beam fixed and directed to pass through said transparent medium and intersect said capillary so that said light beam passes from said transparent medium into said capillary, through said capillary, and back out into said transparent medium after thereby having been twice refracted by said fluid in said fluid bearing capillary; C. detecting the displacement of said twice refracted beam by position sensor means, where said displacement is caused by the refractive index change of said fluid due to said dissolved solute, and said detected displacement is converted into a value proportional to said displacement; D. converting said displacement value into a direct numerical or digital form representative of dn/dc where dn is the refractive index change of said fluid caused by a concentration change dc of said solute within said fluid.
 15. The method of claim 14 where said light beam is oriented with respect to said capillary at an angle slightly less than the critical angle θ_(c) where

    θ.sub.c =sin .sup.-1 (n.sub.s /n.sub.g),

n_(s) is the refractive index of said capillary borne fluid, and n_(g) is the refractive index of said transparent medium.
 16. The method of claim 14 where said fine light beam is of a diameter less than the diameter of said capillary.
 17. The method of claim 14 where said fine light beam is generated by a laser.
 18. The method of claim 14 where said fine light beam may be adjustably fixed at different angles of incidence to said capillary.
 19. The method of claim 14 where said position sensor means is a two element photodiode.
 20. The method of claim 14 where said position sensor means is adjustably fixed so as to intercept said twice refracted light beam passing through said capillary.
 21. The method of claim 14 where said capillary is provided with liquid handling means permitting the injection of fluid therethrough.
 22. The method of claim 14 where said method incorporates a second light beam passing through and parallel to said capillary and said capillary is surrounded by a set of associated coplanar detectors positioned about said transparent medium so that light scattered in the plane of said coplanar detectors is detected thereby.
 23. The method of claim 14 where said fine light beam is monitored by a signal proportional to the incident intensity of said light beam and said displacement value is divided by said signal prior to conversion into said fluid refractive index change.
 24. The method of claim 20 where said fixed position sensor means is adjusted such that the signal produced thereby is null when said dissolved solute is absent from said fluid.
 25. The method of claim 14 where said spherical transparent medium has attached means permitting the passage of a second fine light beam parallel to and through the center of said fluid bearing capillary, said spherical transparent medium being further surrounded by an array of detectors such that said detectors, capillary, and second fine light beam lie in a plane; said detectors detecting light scattered from said capillary borne fluid over a range of scattering angles.
 26. The method of claim 14 where said fine light beam is plane polarized.
 27. A method for measuring the refractive index change of a fluid caused by a change in the concentration of a solute dissolved therein, comprising the steps ofA. providing a fluid bearing capillary lying along a diameter of a spherical transparent medium, said medium being a refractive index greater than that of said capillary borne fluid; B. illuminating said capillary by a coplanar fine light beam fixed and directed to pass through said transparent medium and intersect said capillary so that said light beam passes from said transparent medium into said capillary, through said capillary, and back out into said transparent medium after thereby having been twice refracted by said fluid in said fluid bearing capillary; C. detecting the intensity change of said twice refracted beam by detector means, where said intensity change is caused by the refractive index change of said fluid due to said dissolved solute, and said intensity change is converted into a value proportional to said refractive index change; D. converting said intensity change into a direct numerical form representative of dn/dc where dn is the refractive index change of said fluid caused by a concentration change dc of said solute within said fluid.
 28. The method of claim 27 where said light beam is oriented with respect to said capillary at an angle slightly less than the critical angle θ_(c) where

    θ.sub.c =sin .sup.-1 (n.sub.s /n.sub.g),

n_(s) is the refractive index of said capillary borne fluid, and n_(g) is the refractive index of said transparent medium.
 29. The method of claim 27 where said light beam is of a diameter less than the diameter of said capillary.
 30. The method of claim 27 where said fine light beam is generated by a laser.
 31. The method of claim 27 where said fine light beam may be adjustably fixed at different angles of incidence to said capillary.
 32. The method of claim 27 where said detector means is adjustably fixed so as to intercept said twice refracted light beam passing through said capillary.
 33. The method of claim 27 where said method incorporates a second light beam passing through and parallel to said capillary and said capillary is surrounded by a set of associated coplanar detectors positioned about said transparent medium so that light scattered in the plane of said coplanar detectors is detected thereby.
 34. The method of claim 27 where said direct numerical form of said intensity change of said twice refracted beam is divided by a numerical form derived from a signal proportional to the intensity of said incident fine light beam and the resulting numerical ratio is further converted into a numerical form representative of said fluid refractive index change. 